Many attempts have been made in the past to improve the surface properties of metals in order to widen their applications. An early attempt was to improve corrosion resistance by coating one metal with one or more suitable metals. For instance, base metals have been coated with silver and/or gold, and iron has been coated with tin.
It is also known to use zinc or cadmium to protect iron or steel galvanically against corrosion.
A known process for providing an adherent, protective coating of zinc or zinc compounds on iron or steel is by hot dip galvanizing. This involves immersing the iron or steel object in a bath of molten zinc. However, the degree of protection provided by this process is highly dependent on the bath temperature, immersion time, rate of cooling or subsequent reheating. Moreover, the strength and impact toughness of the substrate is generally reduced, and the zinc coating tends to craze or crack if the hot dip galvanized substrate is subsequently formed by sharp bending.
Processes to electroplate a layer of zinc on steel have also been suggested. However, it is recognized that the protective effect of a zinc coating on steel is mainly sacrificial protection and only partially provided by the formation of an insoluble surface coating. The degree of protection against corrosion depends on the formation of an insoluble basic carbonate film. Any condition which interferes with the formation of this film will lead to rapid attack of the zinc coating and negate the protective effects provided. Further, zinc plating has proven to be unsatisfactory for protection against the corrosive effects of severe industrial environments.
Besides providing corrosion resistance, metals have been coated with cadmium to provide lubricity, solderability, and compatible electrical conductivity. Products coated with cadmium have found application particularly in the aerospace and automobile industries. Cadmium coated steels are used in aircrafts, aerospace fasteners, disc-brake components, radiator hose fittings, door latches and torsion-bar bolts. However, because of the toxicity of cadmium and the potential health hazards resulting therefrom, there are stringent federal and local regulations controlling the use of cadmium. This limits its application and increases its cost.
With the growth in use of metals in many industries, methods of joining metals together have been found to be essential. This led to the use of fasteners such as bolts, screw, springs, pins, etc. Many of these rely upon the use of threaded parts which are subjected to high torque loads to keep the metal components from pulling apart or becoming unfixed.
However, it has been found that when the threaded parts are highly torqued, they become susceptible to corrosion, such as by hydrogen sulfide attack, hydrogen embrittlement, chloride corrosion, stress corrosion cracking and oxidative attack.
To protect against such corrosive attacks, methods of plating or coating the threaded parts have been developed.
It is important that the coating or plating on the threaded parts be very thin so that it does not interfere with the thread make up. It is also important that the coating or plating adhere to the base metal, to provide a low coefficient of friction and protect against corrosive attack.
Methods of coating or plating threaded parts most commonly used today are galvanizing, zinc plating, phosphating, cadmium plating or coating with fluorocarbon polymers. However, these methods suffer from many disadvantages.
As an example, an ASTM B-7 bolt should have a maximum tensile strength of 80,000 lbs. and a usable temperature range of subzero to 600.degree. C.
Galvanizing can only provide a coating with a tensile strength of about 40,000 lbs. The deposited layer is thick and special nuts are required for thread make up. Further, the smallest breakdown in the coating provides sites for accelerated corrosion whereby the nuts and bolts fuse together resulting in extra expenses for removal during maintenance.
Zinc electroplating also provide poor tensile strength and low resistance to corrosion.
Cadmium electroplating can provide a tensile strength of 70,000 lbs. and a low coefficient of friction. However, it only provides moderate protection against corrosion and any breakdown in the coating accelerates corrosive attack. And as stated previously, cadmium is highly toxic with severe environmental implications.
Phosphating can provide good mechanical properties, and act as a substrate for paints and fluorocarbon polymer coatings. However, by itself, phosphating does not provide sufficient protection against corrosion.
Fluorocarbon polymer coatings do provide good corrosion resistance and a low coefficient of friction. However, the usable temperature range is very limited and fluorocarbon polymers tend to flow excessively under stress.
Therefore, there is a critical need for a coating that is resistant to corrosion and will prevent galling, particularly in the oil exploration field.
Galling is a problem encountered frequently in oil and gas exploration. Galling describes a phenomenon when threads of connectors become forged or welded together as a result of having been subjected to high torque loading.
In the oil and gas exploration field increasing depths of drilling have been found to be necessary to obtain much needed energy resources. High temperatures and pressures coupled with aggressively corrosive environments such as hydrogen sulfide, hot boiling chlorides, carbon dioxide gas have compounded the problems of oil exploration and expensive metal alloys have been developed to meet the challenge.
Since it is not uncommon to drill wells of 15,000 feet or deeper, drilling pipes must be threaded together. Further, tool joints, pulsation dampers, blow out preventers, valves, electric measuring devices are used and all of these have threaded connections. The galling of threaded connections have been a severe problem in oil and gas exploration causing increased expenses in time and money.
Attempts have been made to overcome this problem with specially designed pipe threads and coatings such as electroless nickel, hard chromium. However, none of the coatings provide a satisfactory solution since most of the coatings break down under the high stress load required, and some of the coatings such as electroless nickel or hard chromium have uneven throwing power which leads to distortion of threaded parts which are specially designed to have close tolerance.
It has been found that the zinc/silicon/phosphorous deposit according to the present invention is surprisingly effective against corrosion of threaded joints and is particularly suitable for applications in the oil exploration field.
Another serious problem recently encountered is stress corrosion cracking of high strength alloys. These high strength alloys have been used in many different areas from satellites and space vehicles to cars, bridges and nuclear reactors. It is recognized that stress corrosion cracking is related to hydrogen embrittlement or attack by sulfides and chlorides in the environment. Sulfide induced stress corrosion cracking is generally considered to be a result of hydrogen embrittlement. When hydrogen atoms evolve cathodically on the surface of a metal, as a result of corrosion, the presence of hydrogen sulfide causes the hydrogen atoms to stay within the surface of the metal. These hydrogen atoms diffuse to regions of high triaxial tensile stress or regions where the microstructural configuration causes the hydrogen atoms to be trapped. The presence of hydrogen atoms increases the brittleness of the metal. Stress failure has been the major cause of airplane and auto crashes, and flaws in bridges and nuclear reactors. Up to the present, no viable solution to stress corrosion cracking of high strength alloys has been provided.
It has been found further that the zinc/silicon/phosphorus coating according to the present invention can improve the stress corrosion cracking resistance of these high strength alloys.
Wear and friction are also serious problems in metal engine components, where parts are in contact with each other, especially piston rings, cylinders, and auto transmission shafts. The rate of wear is directly related to the amount of friction between two moving components. Lubricants are used to reduce friction. However, in certain cases, lubricant oils may interfere with the function of the parts; furthermore dirty lubricant oils must be changed frequently, thus increasing the cost of operation and present disposal problems. Since the zinc/silicon/phosphorus coating provides a low coefficient of friction, it is particularly useful on metal parts which are in contact with each other.
The recognition of the need to improve resistance to corrosion, galling, wear, and stress corrosion cracking has led to the development of many methods of improving the surface characteristics of metals. One such known method is to "siliconize" metal substrates by exposing the metal substrate to high temperatures, in the range of 800.degree.-1400.degree. C., in an atmosphere of silicon tetrachloride and hydrogen. Alternatively, metal substrates may be siliconized by heating the metal sutstrates in the presence of silicides at a temperature sufficient to cause thermal decomposition of the silicide. Such siliconized metal substrates are found to be highly resistant to oxidative attack, and possess anti-corrosion characteristics.
However, these processes consume extremely high amounts of energy and are difficult to control and impractical.
Research data in phosphorus implantation have been reported to show improved corrosion resistance of stainless steel. However, this process requires expensive and sophisticated processing equipment and at the present time, is impractical for use in production.
Surprisingly, it has been found according to the present invention that a zinc/silicon/phosphorus coating can provide a solution to all of the above problems. The zinc/silicon/phosphorous coating has a low coefficient of friction, equivalent to that provided by cadmium. The coating adheres well and is not destroyed when subjected to full torque and tensile loading. Further, only a layer of 0.2 to 0.3 mil thickness is sufficient to provide excellent corrosion resistance. The coating can be deposited on any conductive substrate, including but not limited to aluminum, titanium, chromium, stainless steels and alloys, high strength metals used by the aerospace industry. Moreover, the coating eliminates galling problems associated with high strength alloy steels.
Another area where the present invention has application is in the plating of difficult to plate metal substrates, such as aluminum, titanium and stainless steel. Up to the present, it is difficult to obtain good adhesion to these metal substrates because of the presence of a film of metal oxide on the surface. The metal oxide film can be removed by immersion in acidic or alkaline solutions. However, the oxide film re-forms immediately when the metal substrate is removed from the de-oxidizing solution. Although methods to improve adhesion are available, these generally involve additional processing steps and increased production costs.
For example, in the production of magnetic recording discs, aluminum is provided with a layer of electroless nickel to provide adhesion for subsequent deposition of a magnetic coating. However, nickel is expensive and it provides a hard surface which is difficult to grind smooth for subsequent processing.
Although stainless steel is easier to plate than aluminum, the formation of an adherent coating by electrodeposition is extremely difficult. In fact, stainless steel is frequently used as the substrate when it is desired to form a coating which can be subsequently removed from the substrate for mechanical testing. The process for electrodepositing a coating on stainless steel involves several pickling steps prior to electrodeposition and, even then, a
Titanium is extremely difficult to plate because of the formation of an extremely stable oxide film, which prevents the formation of an adherent coating. Therefore, the pickling step to remove this stable oxide film is even more crucial, and hydrofluoric acid is often used. Clearly, this is undesirable because of the corrosive nature and the danger of this solution.
It is desirable, therefore, to provide electrodeposition methods for the plating of these metals.
Attempts to electroplate silicon have been made in research laboratories. Unfortunately, these processes require the use of extremely high temperatures or of non-aqueous solvents. The former results in high energy consumption, while the problem of water removal and waste disposal has to be overcome with non-aqueous solvents.
Recently, three patents have issued describing methods of producing inorganic multi-metal polymeric complexes containing hydrophosphide groups in aqueous solutions. U.S. Pat. No. 4,029,747 described a polymeric-metal complex of a non-alkaline metal and an alkali metal in ammonia, for example silicon-sodium in ammonia and aluminum/sodium/calcium complex in ammonia. U.S. Pat. No. 117,088 describes an inorganic polymeric metal complex of non-alkaline metal of Group I-VIII, an alkali metal and a phosphorous compound in aqueous solutions. Specifically, Example 11 discloses a silicon-sodium-phosphorus polymeric complex. U.S. Pat. No. 4,117,099 describes an inorganic polymeric metal complex of non-alkaline metal of Group I-VIII, an alkali metal and a sulfur containing compound.
It is indicated that the polymeric solutions may be useful in plating. Specifically U.S. Pat. No. 4,029,747 suggests that the complexes may be used to plate silicon. U.S. Pat. No. 4,117,088 does not disclose or suggest the plating of silicon, and it was found that a solution prepared according to Example 11 of U.S. Pat. No. 4,117,088 did not produce an electrodeposit of silicon. Surprisingly, the applicants found that when zinc ions were added to the silicon-containing solutions, silicon was co-deposited with zinc. It is to be noted, however, none of the disclosures in these patents suggest that a co-deposit of silicon and phosphorus with zinc by electrodeposition can be obtained.
It is an object of the present invention to provide an economical and practical method to improve the resistance of metals to corrosion, wear, galling and stress corrosion cracking.
It is another object of the present invention to provide a method of electrodepositing a zinc/silicon/phosphorus coating on metal to improve the resistance of the metal to corrosion, wear, galling and stress corrosion cracking.
It is a further object of the present invention to provide an aqueous composition to electrodeposit a zinc/silicon/phosphorus coating on metals including difficult to plate metals, such as aluminum, stainless steel and titanium.
It is another object of the present invention to electrodeposit reliably silicon-comprising coatings.
It is a further object of the present invention to provide a metallic article having a surface coating comprising zinc/silicon/phosphorus.